1. Field of the Invention
The invention relates to an apparatus for optically scanning a recording track of an optical record carrier, said apparatus comprising a radiation source for supplying a scanning beam, and an imaging optical system for imaging the radiation source to form a scanning spot on the recording track.
The recording track may be a track already optically recorded with information, or an optically detectable track in which information is to be written optically. In the first-mentioned case the scanning beam is a read beam and in the second case the scanning beam is a write beam which is intensity-modulated in accordance with the information to be written.
The recording track may be provided on different types of record carriers, such as a tape-shaped or a round disc-shaped record carrier. In the latter case the recording track may be a continuous spiral track comprising a multitude of turns or it may comprise a multitude of separate concentric turns.
2. Description of the Related Art
Examples of the above-mentioned apparatus are a read apparatus for an optical audio or video disc known as "CD" and "Laser-vision" players, respectively, or a combined write and read apparatus for an optical data disc.
In order to achieve that the scanning spot is always correctly positioned with respect to a track portion to be scanned, the optical scanning apparatus of the type described comprise a tracking servo-system. In such a system the position of the centre of the scanning spot with respect to the centre line of the scanned track portion is detected and the position of the scanning spot is corrected in accordance with the tracking error signal obtained.
As described in inter alia U.S. Pat. No. 4,063,287 a tracking error signal may be generated by periodic movement of the scanning spot at a given frequency transversely to the track direction. Due to the periodical displacement of the scanning spot the radiation plane in which the recording track is situated is periodically modulated. As a result, the output signal of a radiation-sensitive detector arranged in the path of the radiation has a periodically varying component whose amplitude and phase, related to the movement of the scanning spot, represent the magnitude and the direction of a deviation of the scanning spot position with respect to the scanned track portion.
In the apparatus described in U.S. Pat. No. 4,063,287 the periodical movement of the scanning spot is produced by means of a mirror arranged in the path of the scanning beam, which mirror is periodically pivoted about an axis which is effectively parallel to the direction of the scanned track portion. The average position of the scanning spot can be corrected by setting, for example, the central or nominal position of this mirror by means of the tracking error signal which has been generated. Such a tracking servo-system, in which moving elements are used, has a limited bandwidth.